--[[ $Id: CallbackHandler-1.0.lua 26 2022-12-12 15:09:39Z nevcairiel $ ]]
local MAJOR, MINOR = "CallbackHandler-1.0", 8
local CallbackHandler = LibStub:NewLibrary(MAJOR, MINOR)

if not CallbackHandler then
    return
end -- No upgrade needed

local meta = { __index = function(tbl, key)
    tbl[key] = {}
    return tbl[key]
end }

-- Lua APIs
local securecallfunction, error = securecallfunction, error
local setmetatable, rawget = setmetatable, rawget
local next, select, pairs, type, tostring = next, select, pairs, type, tostring

local function Dispatch(handlers, ...)
    local index, method = next(handlers)
    if not method then
        return
    end
    repeat
        securecallfunction(method, ...)
        index, method = next(handlers, index)
    until not method
end

--------------------------------------------------------------------------
-- CallbackHandler:New
--
--   target            - target object to embed public APIs in
--   RegisterName      - name of the callback registration API, default "RegisterCallback"
--   UnregisterName    - name of the callback unregistration API, default "UnregisterCallback"
--   UnregisterAllName - name of the API to unregister all callbacks, default "UnregisterAllCallbacks". false == don't publish this API.

function CallbackHandler.New(_self, target, RegisterName, UnregisterName, UnregisterAllName)

    RegisterName = RegisterName or "RegisterCallback"
    UnregisterName = UnregisterName or "UnregisterCallback"
    if UnregisterAllName == nil then
        -- false is used to indicate "don't want this method"
        UnregisterAllName = "UnregisterAllCallbacks"
    end

    -- we declare all objects and exported APIs inside this closure to quickly gain access
    -- to e.g. function names, the "target" parameter, etc


    -- Create the registry object
    local events = setmetatable({}, meta)
    local registry = { recurse = 0, events = events }

    -- registry:Fire() - fires the given event/message into the registry
    function registry:Fire(eventname, ...)
        if not rawget(events, eventname) or not next(events[eventname]) then
            return
        end
        local oldrecurse = registry.recurse
        registry.recurse = oldrecurse + 1

        Dispatch(events[eventname], eventname, ...)

        registry.recurse = oldrecurse

        if registry.insertQueue and oldrecurse == 0 then
            -- Something in one of our callbacks wanted to register more callbacks; they got queued
            for event, callbacks in pairs(registry.insertQueue) do
                local first = not rawget(events, event) or not next(events[event])    -- test for empty before. not test for one member after. that one member may have been overwritten.
                for object, func in pairs(callbacks) do
                    events[event][object] = func
                    -- fire OnUsed callback?
                    if first and registry.OnUsed then
                        registry.OnUsed(registry, target, event)
                        first = nil
                    end
                end
            end
            registry.insertQueue = nil
        end
    end

    -- Registration of a callback, handles:
    --   self["method"], leads to self["method"](self, ...)
    --   self with function ref, leads to functionref(...)
    --   "addonId" (instead of self) with function ref, leads to functionref(...)
    -- all with an optional arg, which, if present, gets passed as first argument (after self if present)
    target[RegisterName] = function(self, eventname, method, ... --[[actually just a single arg]])
        if type(eventname) ~= "string" then
            error("Usage: " .. RegisterName .. "(eventname, method[, arg]): 'eventname' - string expected.", 2)
        end

        method = method or eventname

        local first = not rawget(events, eventname) or not next(events[eventname])    -- test for empty before. not test for one member after. that one member may have been overwritten.

        if type(method) ~= "string" and type(method) ~= "function" then
            error("Usage: " .. RegisterName .. "(\"eventname\", \"methodname\"): 'methodname' - string or function expected.", 2)
        end

        local regfunc

        if type(method) == "string" then
            -- self["method"] calling style
            if type(self) ~= "table" then
                error("Usage: " .. RegisterName .. "(\"eventname\", \"methodname\"): self was not a table?", 2)
            elseif self == target then
                error("Usage: " .. RegisterName .. "(\"eventname\", \"methodname\"): do not use Library:" .. RegisterName .. "(), use your own 'self'", 2)
            elseif type(self[method]) ~= "function" then
                error("Usage: " .. RegisterName .. "(\"eventname\", \"methodname\"): 'methodname' - method '" .. tostring(method) .. "' not found on self.", 2)
            end

            if select("#", ...) >= 1 then
                -- this is not the same as testing for arg==nil!
                local arg = select(1, ...)
                regfunc = function(...)
                    self[method](self, arg, ...)
                end
            else
                regfunc = function(...)
                    self[method](self, ...)
                end
            end
        else
            -- function ref with self=object or self="addonId" or self=thread
            if type(self) ~= "table" and type(self) ~= "string" and type(self) ~= "thread" then
                error("Usage: " .. RegisterName .. "(self or \"addonId\", eventname, method): 'self or addonId': table or string or thread expected.", 2)
            end

            if select("#", ...) >= 1 then
                -- this is not the same as testing for arg==nil!
                local arg = select(1, ...)
                regfunc = function(...)
                    method(arg, ...)
                end
            else
                regfunc = method
            end
        end

        if events[eventname][self] or registry.recurse < 1 then
            -- if registry.recurse<1 then
            -- we're overwriting an existing entry, or not currently recursing. just set it.
            events[eventname][self] = regfunc
            -- fire OnUsed callback?
            if registry.OnUsed and first then
                registry.OnUsed(registry, target, eventname)
            end
        else
            -- we're currently processing a callback in this registry, so delay the registration of this new entry!
            -- yes, we're a bit wasteful on garbage, but this is a fringe case, so we're picking low implementation overhead over garbage efficiency
            registry.insertQueue = registry.insertQueue or setmetatable({}, meta)
            registry.insertQueue[eventname][self] = regfunc
        end
    end

    -- Unregister a callback
    target[UnregisterName] = function(self, eventname)
        if not self or self == target then
            error("Usage: " .. UnregisterName .. "(eventname): bad 'self'", 2)
        end
        if type(eventname) ~= "string" then
            error("Usage: " .. UnregisterName .. "(eventname): 'eventname' - string expected.", 2)
        end
        if rawget(events, eventname) and events[eventname][self] then
            events[eventname][self] = nil
            -- Fire OnUnused callback?
            if registry.OnUnused and not next(events[eventname]) then
                registry.OnUnused(registry, target, eventname)
            end
        end
        if registry.insertQueue and rawget(registry.insertQueue, eventname) and registry.insertQueue[eventname][self] then
            registry.insertQueue[eventname][self] = nil
        end
    end

    -- OPTIONAL: Unregister all callbacks for given selfs/addonIds
    if UnregisterAllName then
        target[UnregisterAllName] = function(...)
            if select("#", ...) < 1 then
                error("Usage: " .. UnregisterAllName .. "([whatFor]): missing 'self' or \"addonId\" to unregister events for.", 2)
            end
            if select("#", ...) == 1 and ... == target then
                error("Usage: " .. UnregisterAllName .. "([whatFor]): supply a meaningful 'self' or \"addonId\"", 2)
            end

            for i = 1, select("#", ...) do
                local self = select(i, ...)
                if registry.insertQueue then
                    for eventname, callbacks in pairs(registry.insertQueue) do
                        if callbacks[self] then
                            callbacks[self] = nil
                        end
                    end
                end
                for eventname, callbacks in pairs(events) do
                    if callbacks[self] then
                        callbacks[self] = nil
                        -- Fire OnUnused callback?
                        if registry.OnUnused and not next(callbacks) then
                            registry.OnUnused(registry, target, eventname)
                        end
                    end
                end
            end
        end
    end

    return registry
end


-- CallbackHandler purposefully does NOT do explicit embedding. Nor does it
-- try to upgrade old implicit embeds since the system is selfcontained and
-- relies on closures to work.

